The present invention relates to an image display apparatus and, more particularly, to a head- or face-mounted image display apparatus that can be retained on the observer's head or face.
As a conventional image display apparatus, Japanese Patent Application Unexamined Publication (KOKAI) No. 8-313829, filed by the present applicant, is known. This image display apparatus projects an image displayed by an image display device into an observer's eyeball through a modified prism formed from three surfaces as an ocular optical system. In the modified prism, the three surfaces are each decentered with respect to the observer's visual axis, and a space formed by the three surfaces is filled with an optically transparent medium having a refractive index of about 1.5. Let us assume the three surfaces of the modified prism as follows: A surface through which light emitted from the image display device enters the modified prism is a first surface; a reflecting surface opposite to the observer's eyeball is a second surface; and a surface close to the observer's eyeball is a third surface. Light rays emitted from the image display device enter the modified prism while being refracted through the first surface. The rays are internally reflected by the third surface toward the second surface. The reflected rays are internally reflected by the second surface toward the third surface. Then, the reflected rays exit from the modified prism while being refracted through the third surface and reach the observer's eyeball.
In a modified prism formed from decentered surfaces as stated above, when reflection takes place in the prism at an angle not exceeding the critical angle, the reflecting surface needs to be provided with a coating of a metal having a high reflectivity, e.g. aluminum. Moreover, there is a portion containing an area outside the effective area. At such a portion, not only transmission but also internal reflection or other similar phenomenon occurs. Accordingly, light rays emitted from the image display device or light rays externally entering the modified prism include not only light traveling along the above-described normal path of display light but also light traveling along some other ray path to reach the observer's eyeball. There is also light that reaches the observer's eyeball by transmission or reflection at an area outside the effective area. These light rays are unnecessary for the observer and recognized as ghost or flare, causing resolution and MTF (Modulation Transfer Function) to be degraded. Thus, the unwanted light has an adverse effect on the image for observation.
Under these circumstances, the present applicant proposed, in Japanese Patent Application Unexamined Publication (KOKAI) No. 9-65245, to provide a light-blocking device between an ocular optical system and an observer's eyeball in an image display apparatus of the type described above to prevent light from an image display device from traveling along an optical path other than the normal path of display light and entering the pupil of the observer's eye as ghost light, which would otherwise cause a ghost image or the like to be formed outside the display area, and also prevent external light from the outside world from being reflected by a surface of the modified prism and entering the observer's pupil as flare or ghost, which would otherwise cause the image display performance to be degraded.
However, even in this case, it is not always possible to satisfactorily remove unwanted light caused by reflection at a reflecting area outside the effective area of the second surface of the above-described modified prism, which becomes flare or ghost, and also unwanted external light that enters the modified prism through a transmitting area outside the effective area of the first surface, which faces the image display device, and forms flare or ghost.